Fluid meters are widely used for most kinds of fluids in different application areas. Fluid meters are for example used in fuel dispensing pumps for retail sale of motor fuel, providing a means for measuring the quantity dispensed from the pump. The measured volume is typically communicated to a register, displaying the dispensed volume and the price.
A fluid meter commonly used for fuel dispensers is shown by Ainsworth, U.S. Pat. No. 2,756,726. In this disclosure a meter having a multiple piston hydraulic motor is used. Fluid is allowed to enter cylinders and cause reciprocation of the pistons. The pistons are connected to a shaft that will rotate as an effect of the reciprocation. A rotary valve, coupled to the shaft, admits liquid to the cylinders or permits flow to the outlet connections, in proper timed relation. The fluid meter utilizes what may be termed “hypothetical” cylinders, mechanically and hydraulically cooperating with the cylinders and pistons which are structurally existent.
This is accomplished by arranging the ports and the rotary valve so as to sequentially admit fluid to both the crankcase and the ends of the cylinders at the same time as fluid is withdrawn from the cylinders. The fluid volume admitted to, or withdrawn from, the crankcase is the algebraic sum of the volume withdrawn from, or admitted to, the cylinders. Two pistons, actuated through the valve mechanism, advantageously 120 degrees out of phase, thus perform the work equivalent of three pistons. This reduces the actual number of cylinders required for a given capacity, reduces internal friction and pulsation, and achieves smoother operation. The two pistons are attached via connecting rods to a crankshaft with a radially offset crank pin. The crank pin engages a yoke in each connecting rod so that the reciprocating movement of the two pistons is transformed into a rotary motion of the crankcase in accordance with the Scotch Yoke type principle. To accomplish the phase differences between the pistons, the two physical cylinders are oriented with an angle of 120 degrees between their respective centre axis.
The Ainsworth fluid meter has several drawbacks, as e.g. the requirement of special piston guide barrels, the arrangement of cylinders and guide barrels is difficult to mould or cast and machine, and the register is driven by a shaft extending through the meter housing with accompanying risk of leakage.
A similar fluid meter is disclosed by Spalding, U.S. Pat. No. 5,686,663 and WO 98/49530. This fluid meter aims at eliminating the drawbacks of the Ainsworth fluid meter. Thus, the two angled cylinders of Ainsworth are aligned along a common centre axis to eliminate the bulky construction of Ainsworth. To accomplish the same piston reciprocity, the crankshaft is modified with an extra crank arm. The in-line construction is advantageous when several meters have to be mounted in one dispenser, which is the normal case in most modern fuel dispensers.
The Spalding fluid meter, however, is not without some drawbacks. As the fluid meter is used, wear will affect the parts within the fluid meter. The wear on the parts of the fluid meter may lead to a fluctuation in stroke length, which possible over time will result in an error in the volume readings of fluid meter.